Research status and perspective for stony-hard peach

Abstract: Peach is the 3rd largest deciduous fruit in China, which is commercially planted almost all around the country. Traditionally, most of the cultivars planted in China are melting flesh type whose flesh becomes softening very quickly at the end of fruit development and the fruits are difficult to deliv- er to far markets. In recent years, development of cultivars with good keeping quality has become one of the most important breeding objectives pursued by many peach breeders. Peach is a typical kind of respiratory climactic fruit. Normally, the ethylene production rises gradually with the progress of peach fruit ripening and reaches a peak finally. Owing to the action of ethylene, PG enzymes (including exo- and endo-PG) are catalyzed, which resulted in cell wall degradation. Then, the firmness of the peach fruits declines dramatically and the fruits become soft and juicy soon. However, there are also some other kinds of peach fruit textures. Stony-hard peach is a kind of fruits in which the production of ethylene is blocked, which results in that the fruit can keep hard even on-tree or during post-harvested period. This nature makes it a very good material to make up the shortage of melting flesh type. Stony-hard flesh type was originally reported in 1976 by Yoshida. However in China, there were no reports about this kind of peach until recent years.‘Jingyu’was the first stony-hard peach cultivar selected by Bei- jing Forestry and Pomology Institute in 1970s. It was widely used in peach breeding programs in China, not for the fruit texture at the beginning but for the carrier of g gene for nectarine breeding. There are many stony-hard peach/nectarine cultivars released and commercially planted in China in recent years, such as CN 18, CN 20 and CP 8 bred by Zhengzhou Fruit Research Institute of Chinese Academy of Ag- ricultural Sciences;‘Ruiguang’‘Meiyu’and‘Huayu’bred by Beijing Institute of Forestry and Pomol- ogy; and‘Xiacui’bred by Jiangsu Academy of Agricultural Sciences. In addition, there were also some breeders or institutes from Europe, USA, Japan and so on were engaged in several programs for stony- hard peach breeding and have released some cultivars for commercial production, such as‘Ghiaccio’series‘, Yumyeong’‘, Manami’, etc.. Genetically, the stony-hard trait is controlled by a single recessive gene (hd), which is epistatic to the melting/non-melting character and inherited independently. Many lit- eratures showed that the stony-hard texture was resulted from a mutation of PpYUC11 gene, belonging to YUCCA gene family, which encodes a flavin monooxygenase-like gene. In melting flesh peach fruits,PpYUC11 catalyzes the progress of IAA biosynthesis, and then PpACS1 is induced and catalyzes the production of ethylene, which leads to the softening of peach fruits finally. RT-PCR detection showed that PpYUC11 gene expressed abundantly in melting or non-melting flesh peach fruits during mature stage but undetectable in stony-hard peach fruit. Analysis of the gene structure indicated there existed a 2.1 kb insertion upstream of the start codon of the PpYUC11 gene in stony-hard peach, leading to the losing of function. Based on the researches, a molecular marker has been developed to distinguish stony- hard peach from other flesh textures of peach fruit. By detecting some germplasm resources and culti- vars, we have found some good resources containing hd gene, such as‘CN5’‘Golden Honey 3’‘CP5’‘Yuhualu’‘Baihua’‘NJC83’, etc., which can be used for stony-hard peach breeding.‘CN 15’ ‘CN20’‘Xiacui’‘Qingwang’, and several other cultivars were verified as stony-hard texture with hdhd genotype. Stony-hard peaches or nectarines were welcomed by some farmers and logistics industry for their good keeping quality. But there are still some problems, such as lack of fragrance, relatively less prominent flavor, post-harvest diseases, etc., which need to be taken into account in future from multiple ways including breeding, cultivation and post-harvest handling